The CD24 Protein Inducible Expression System Is an Ideal Tool to Explore the Potential of CD24 as an Oncogene and a Target for Immunotherapy in Vitro and in Vivo

CD24 is a cell surface, heavily glycosylated glycosylphosphatidylinositol-anchored mucin-like protein that is overexpressed in various human malignancies. To accurately analyze CD24 function and dissect its biological role in a defined genetic background, it is critical to tightly regulate its expression and be able to turn it on/off in a restricted environment and at a specific time, rho tetracycline-induced expression system is most promising as it exhibits such regulation, lack of pleiotropic effects, and high and rapid induction levels. To evaluate the oncogenic and immunotherapeutic potential of CD24 by applying the Tet-On system, the human CD24 gene was cloned downstream to two tetracycline operator sequences, resulting in pCDNA4/TO-CD24, which was then transfected into tetracycline (Tet) repressor-expressing cells (293T-REx), allowing tight on/off regulation, thereby resulting in a very low background or leaky CD24 expression. Selected clones were chosen for further studies and characterized in vitro and in vivo, and several treatment modalities were examined. In addition, the role of CD24 in promoting cell proliferation and tumor growth was studied. The tetracycline-dependent system was successfully implemented. Tetracycline treatment induced CD24 expression in a dose- and time-dependent fashion, which was abrogated following treatment with anti-CD24 monoclonal antibodies (mAbs). CD24-induced expression led to an increased proliferation rate that was inhibited by mAb treatment. In vivo, significantly larger tumors were developed in tetracycline-fed mice. The CD24 Tet-On system is a good model to unravel the role and underlying CD24 pathogenesis in vivo. This valuable tool allows the successful study of novel treatment options, whose effectiveness depends on the CD24 expression level. This set of experiments supports CD24 oncogenic properties.